1. Technical Field
The invention is in the field of electro-optic modulators of light wave intensity. More particularly, the invention relates to using thin film ferroelectric materials in a Fabry-Perot etalon to provide polarization-insensitive modulation of reflected and transmitted light at high modulation frequencies.
2. Related Art
A number of methods or devices have been proposed to modulate the intensity of light. Most of them are not suitable for producing useful large-scale hybrid, integrated-with-silicon, circuits. The closest related art to the present invention uses ferroelectric materials whose indices of refraction change when an electric field is applied, but at a different rate for different light polarizations. That is, the ordinary and the extraordinary index change at a different rate with applied voltage, i.e., a field induced birefringence occurs. This structure is similar to an asymmetric Fabry-Perot etalon with a back surface reflecting electrode and a front surface partially transmitting electrode. Light which has been linearly polarized so that it has equal amplitude p-waves parallel to the incident plane and s-waves perpendicular to the incident plane are incident on and, after undergoing multiple reflections inside, reflected by the etalon. The reflected light is passed through a second linear polarizer which is used as an analyzer. In the absence of an electric field applied to the ferroelectric material, the intensity of the light exiting the second polarizer will be a function of several parameters including the type and thickness of the ferroelectric material and the reflectivity of electrodes. When an electric field is applied to the ferroelectric material, the relative optical phase of the p- and s-waves will be changed such that the intensity of light exiting the second polarizer will change. At the proper angle of incidence, the large field-induced birefringence in some ferroelectric materials, makes it possible to obtain a large ratio between light intensities with and without an electric field.
Transmission mode devices have been proposed using coplanar electrodes on PLZT plates. To reduce the high driving voltage, laminar assemblies with buried combs of electrodes have been proposed. All of these approaches have one major disadvantage. The light must be linearly polarized by a first polarizer, resulting in a 50% loss in light intensity for unpolarized light, and a second polarizer used to detect the intensity modulation. Accordingly, it is one object of the present invention to produce a modulator which does not require polarized light.
Other related devices include U.S. Pat. No. 4,786,128 to Birnbach, issued Nov. 22, 1988, which discloses a dielectric mirror comprised of a stack of alternating layers of non-electro-optic dielectric material and electro-optic material of a different index. Preferably, the stack contains nine to eleven layers for the well known reason that more layers yield higher reflectivity, here, 95%. However, in order to avoid large voltages, the voltage applied to the electro-optic material must be transverse necessitating depositing an electrode with its index of refraction matched to the index of the non-electro-optic material. Manufacturing may be costly and the need to make electrical contact to a vertical stack of electrodes may make it hard to produce as a hybrid integrated circuit.
U.S. Pat. No. 5,037,169 to Chun, issued Aug. 6, 1991, discloses a Fabry-Perot etalon with a compound semiconductor, for example, aluminum gallium arsenide. This can be used as an optical switch by changing the index of refraction in a variety of ways including optical pumping, using an external control voltage, injection of electric current, and temperature ramping. Because an appreciable index change occurs only for light wavelengths in the vicinity of the absorption band of the semiconductor materials, modulators of this kind are limited to a narrow range of wavelengths in the near infrared. Moreover, this device seems to be limited in useable angles of incidence.
U.S. Pat. No. 5,425,115 to Wagner, issued Jun. 13, 1995, discloses a polarization insensitive optical switch using liquid crystal material in the cavity of a Fabry-Perot etalon which uses dielectric mirrors. This device can overcome the limitation on light wavelengths, but liquid crystal response times are inherently slow and PLZT was also proposed. While it is not too difficult to insert liquid crystal material into a cavity or use a polished PLZT plate sandwiched between the dielectric mirrors, neither device is easily made part of a hybrid integrated circuit and, as noted, the PLZT driving voltage will be high. Moreover, the free spectral range will be narrow and the requirements on light collimation high. Low voltages, desirable with hybrid integrated circuits using PLZT, requires the deposition of thin films on dielectric mirrors which was heretofore unknown.
Accordingly, objects of the current invention are to produce a device which can modulate the intensity of unpolarized monochromatic light of any wavelength from the visible to mid-infrared without requiring polarizers while having a low operating voltage, a wide modulation bandwidth, viz., dc to about one GHz, and can be produced as a hybrid integrated circuit at low cost.
A solid state device, which is similar to a Fabry-Perot etalon, is used to modulate the intensity of reflected or transmitted light by modulating with an external voltage the optical thickness of a thin film ferroelectric placed in an etalon cavity. The device is constructed by selecting a generally planar supporting substrate, preferably silicon or sapphire in order to be compatible with silicon integrated circuits. A dielectric stack consisting of alternating layers of different index of refraction materials, also specifically selected to be compatible with later growth of the thin film ferroelectric, is deposited thereon to form a partially reflective and partially transmitting mirror, followed by a transparent electrically conductive layer. The thin film ferroelectric is deposited on the conductive layer, followed by a second transparent conductive layer and a second dielectric stack. Leads are connected to the conductive layers and in turn to a voltage generator. In one version of the invention, the functions of both the second (top) electrically conductive layer and dielectric stack are fulfilled by using a semi-transparent conducting film. In another version, the functions of both the first (bottom) electrically conductive layer and dielectric stack are also fulfilled by using a, preferably, highly reflective conducting film.
The devices can use low voltages to control the reflected and, in some versions transmitted, intensity of incident light beams at high frequencies in a continuous manner. The light wavelenghs are not limited to a narrow range, but, with present materials can cover the near ultra-violet to the near-infrared. Further, being able to use thin film techniques to make the ferroelectric material as opposed to starting with the only previously available bulk ceramic plates, should make production of hybrid integrated circuits practical, including the possibilities of making cost-effective arrays.